Method for pollution emission reduction from glass melting furnaces

ABSTRACT

A method for reducing pollution emissions from a glass melting furnace using electrostatic granular bed (EGB) technology. The granules in the EGB filter are themselves formed from an alkaline earth metal material. The granules react with sulfur compounds in the exhaust gas and form a layer of alkaline earth metal sulfates and sulfites in the granules. Simultaneously alkali metal-containing particles are deposited on the granules. The accumulated layers of pollutants are easily removed by mechanical agitation.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 07/998,035filed Dec. 29, 1992, now abandoned, which is a continuation-in-part ofapplication Ser. No. 07/945,721 filed Sep. 21, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The manufacture of glass involves the heating of glass batch materialsto high temperatures (approx. 1500° C.) in order to melt and homogenizethe various constituent components. This process is generally carried infurnaces heated directly by the combustion of some sort of fossil fuel.By far, the most common type of glass manufactured is termed soda-limeglass, used for windows, tableware, bottles, etc. This glass ischaracterized by its batch material, consisting primarily of silicasand, limestone, soda ash and salt cake. The present invention isconcerned primarily with this glass type, but other glass types, such asborosilicate, alumina-silica, lead, etc., also lie within the scope ofthe invention.

Particulate emissions from glass furnaces consist primarily of submicroncondensates of alkali metal sulfates that result from a combination of agas such as sulfur oxide and an alkali metal such as sodium. To a lesserextent, potassium is also present in furnace exhaust gases. Collectionand handling of these particulates is made difficult because of theirlow bulk density, hygroscopic tendency, and acidic potential. Thegaseous emissions of primary concern are sulfur oxides. These arederived primarily from sulfur materials added with the glass batch (saltcake) and from sulfur in the fossil fuel used to heat the furnace. Thebulk of the emissions are in the form of SO₂, but a significant portionare in the form of SO₃. Other pollutants which may be present dependingupon the specific glass type are boron, fluorine, chlorine, and leadoxide dust. Reduction of all these emissions are being mandated byincreasingly stringent government regulations.

A recent development in pollution control is electrostatic granular bedfilter (EGB) technology, as exemplified by U.S. Pat. No. 4,338,113.Pollutants are adhered to electrically charged granular material asexhaust gases pass over the granules.

This technology has been found to be very effective in removingpollutants. However, a problem occurs in that over a period of time, theconductivity of the granules increases, which correspondingly decreasestheir ability to trap pollutants. This greatly reduces the useful lifeof the granules and necessitates down time to replenish the granular bedwith fresh material. Hence, while EGB technology provides effectivepollution abatement, there remains a need in the art for prolonging theuseful life of granules used in EGB technology.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method forreducing pollution emissions from a glass melting furnace exhaust streamusing electrostatic granular bed filter technology.

It is another object of the invention to provide a method, as above,which prolongs the useful life of granules used in the electrostaticgranular bed filter by maintaining a high electrical resistivity in thegranular bed over a long period of time.

These objects are achieved by, in a first embodiment, an electrostaticgranular bed filter process in which an exhaust stream from a glassmelting furnace is injected with a sorbent dust or powder having highresistivity and then is passed over a moving bed of granules in aprereactor. The dust or powder, for example, an alkalineearth-containing component, reacts with sulfur in the exhaust stream toform the corresponding sulfate or other salt which is deposited on thegranules, forming a layer thereon. The exhaust stream is then routed toan electrostatic granular bed filter containing granules similar tothose in the prereactor. Sodium sulfate and/or other salts formed fromthe exhaust gases are deposited on the granular bed of the EGB filter,forming a layer thereon.

Periodically, the two granular beds are randomly mixed together andgranules from the mixture are returned to the prereactor and the EGBfilter. Thus, over time the granules in the EGB filter contain a mixtureof high resistivity compounds as well as salts having low resistivityformed from the exhaust gases. The presence of the high resistivitysalts prevents a decrease in resistivity of the EGB bed.

In another embodiment of the invention, the granules are comprised of analkaline earth metal material. Thus, the granules themselves react withSO₃ and SO₂ gases in the exhaust stream, thereby obviating the need fora separate prereactor. Simultaneously, NaSO₄ and other alkali metalsalts are electrostatically deposited on the granules. Because the molarvolume of alkaline earth metal sulfates and sulfites is greater thanthat of the alkaline earth metal oxide, hydroxide, or carbonategranules, the sulfates and sulfites are easily removed along with thedeposited alkali metal sulfates and sulfites. Various mechanicalagitation methods can be used to remove the deposited material. Thethus-cleaned alkaline earth metal granules, having a high resistivity,can be reused.

BRIEF DESCRIPTION OF THE DRAWINGS

For a full understanding of the invention, the following detaileddescription should be read with reference to the drawings, wherein:

FIG. 1 is a schematic view of a first embodiment of the invention;

FIG. 2 is a plot of electrical resistivity vs. temperature for an EGBfilter;

FIG. 3 is an illustration of granules coated with sodium sulfate;

FIG. 4 is an illustration of granules coated with a layer of sodiumsulfate;

FIG. 5 is a schematic view of a second embodiment of the invention; and

FIGS. 6-8 are illustrations of calcium carbonate granules uncoated andcoated with sodium sulfate and calcium sulfate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention resolves the problem of low granule electricalresistivity in EGB filters due to the formation of low resistivitylayers, and also results in a system that simultaneously reducespollutants such as SO₂ and SO₃.

In a first preferred embodiment, as shown in FIG. 1, an electrostaticgranular bed filter apparatus is indicated generally by the number 10.Exhaust gases from the glass melting furnace and from the regenerator orrecuperator (not shown), are first passed through a gas cooling unit 11using water injection. Water evaporation cools the gases to the desiredrange, generally from about 150° C. to about 500° C., preferably fromabout 400° to about 450° C. The gases then enter a prereactor moving bedunit 12 which is a circularly symmetric cylindrical bed of durablegranules moving downward by gravity. Gas flows through the bed in ahorizontal, radially outward direction. A suitable alkaline earth metalbased dust or powder is air injected into the inner region 13 of theprereactor via an injection blower 14. Suitable alkaline earth metalmaterials are CaO, Ca(OH)₂, CaCO₃, MgO, Mg(OH)₂, MgCO₃, etc. Other highresistivity materials can also be used, such as SiO₂, Al₂ O₃ and thevarious ferrous oxides, however, these latter materials are lesspreferred because they do not react with sulfur compounds in theprereactor. The dust or powder is metered into the injection blower 14by a feeder 15. The dust or powder is injected into the prereactor insufficient quantities so that SO₃ in the flue gas will be neutralizedand form the corresponding sulfate (e.g. CaSO₄). Generally, the amountof dust or powder injected is based on the stoichiometric ratio of thealkaline earth metal to sulfur in the exhaust gases. The ratio can varyfrom about 0.5 to about 4.0, and preferably from about 1.0 to about 2.0.Additional alkaline earth metal may be injected so that SO₂ will bereacted to form the corresponding sulfite. These reactions will takeplace somewhat when the powder is initially contacted with the flue gasin the inner region 13 of the prereactor 12, but primarily in theprereactor bed 16 itself where the alkaline earth metal has beendeposited onto the granule surface.

Granules are moved through the prereactor bed 16 at sufficient velocityso that the injected powder will not cause unstable increases in fluegas pressure drop. Cleaned granules are supplied at the top of themoving bed, while granules with collected powder are removed at thebottom.

Flue gases leave the prereactor 12 and may pass through a second gascooling unit 17 where gases are cooled by water injection andevaporation. This step may be required if it is desired to operate theelectrostatic granular bed filter 18 at a temperature different from theprereactor bed 16. Gases are then passed through an electrostaticpreionizer 19 where a corona discharge is formed in the gas by applyinga high voltage to a cathode electrode (not shown). The ions in thecorona attach to the dust particles in the gas and give them a netelectric charge.

Gases carrying the charged dust particles are then passed through theelectrostatic granular bed filter 18 which includes a durable granularmedia 35 retained between electrically grounded perforated or louveredstructures 36 and 37. A high voltage electrode 38 inside the bedelectrically polarizes the granules. Charged dust particles in the gasare attracted to and are deposited onto the surface of the granules.Cleaned gas exits the filter via outlet 20 and a fan 21 is used toprovide the underpressure to draw the gases through all the previouslymentioned equipment. Gases are then exhausted to the atmosphere.

Granules in the filtration bed 22 of the filter 18 are eitherperiodically or continuously moved through the bed by gravity, thefrequency or rate of motion determined by necessity to maintain stablegas pressure drop across the filter. Granules are then cleaned of dustexternally and returned to the filter 18. The embodiment of FIG. 1illustrates a pressure blower 24 to draw a relatively small portion ofthe cleaned flue gases from the discharge of the fan 21. Pressurizedgases are directed through the venturi ejector 25 which includes anopening in the venturi nozzle to allow granules from filtration bed 22to flow into the pressurized gas stream in pipe 26. These granules arethen pneumatically conveyed through pipe 26 and in the process, dust isremoved from the granules and entrained in the conveying gas. Thegranules are separated from the gas in the separation chamber 27 andfall by gravity into charge hopper 28. Conveying gases and entraineddust are directed to cyclone 29 or equivalent separator where dust isinertially separated. Underpressure to pull gases through the cyclone isprovided by cyclone fan 30 and gases are returned to the main flue gasstream at some suitable point, preferably upstream of electrostaticpreionizer 19. In this way, any residual dust in the conveying gas willbe captured in the filtration bed 22. Cleaned granules are returned fromthe charge hopper 28 to the filtration bed 22 by gravity flow throughinfeed pipe 31.

Granules from the prereactor bed 12 are also cleaned and recycled.However, the rate of granule cleaning from this bed will besignificantly greater than that for filter bed 22, due to the fact thattypically, the amount of high resistivity material injected into theprereactor 12 is several times greater than the rate of dust collectionin the filter bed 22. In a preferred embodiment, a mechanical screen 32is used to separate dust from granules and a bucket elevator 33 is usedto lift the granules. Granules are discharged from the bucket elevatorinto charge hopper 28 and are returned to prereactor bed 12 via infeedpipe(s) 34. The cleaned granules from filter module 18 are co-mingledwith cleaned granules from prereactor bed 12 before they arerespectively reintroduced to the beds. This is conveniently done byutilization of one charge hopper for both beds.

The granule bed of the prereactor is from about 0.1 to about 0.3 metersthick and the bed of granules in the EGB filter is between about 0.5 andabout 1.0 meters thick. The granule migration velocity in the prereactoris from about 10 to about 100 meters/hr while that in the EGB is fromabout 0.2 to about 2 meters/hr.

The granules are co-mingled because it has been found that if granulesutilized in filter bed 22 of module 18 for capture of sodium sulfate(Na₂ SO₄) dust particles are subsequently utilized in prereactor bed 12for capture of CaO (lime) or other high resistivity materials andsubsequently reacted with SO₃ (and possibly SO₂) to form CaSO₄ dust,then the solid layer formed on the granule surface will be a mixture ofNaSO₄ and CaSO₄ (or other alkaline earth metal sulfate). The resultingelectrical resistivity of the coated granule will be a factor of ten toa hundred times greater than that exhibited when a relatively purecoating of Na₂ SO₄ is allowed to form, as would be the case if granuleswere not co-mingled. This then allows the economical application of avoltage to the filter bed which in turn produces polarization of bedgranules and efficient capture of submicron dust particles.

In order for the EGB filter technology to properly function, a highvoltage must be maintained between the high voltage electrode 38 and thegrounded inner louvers 36 and grounded outer louvers 37, the spacebetween the electrode 38 and the louvers 36 and 37 being filled withgranular media. Generally, the electric field strength between theelectrodes must be a minimum of 5×10⁴ volts/meter for effectiveoperation. The granular media is chosen to have good electricalinsulating properties, so that excessive electrical power is notrequired to maintain this high voltage. For typical design parameters, abulk electrical resistivity of at least 1×10⁶ ohm-m is required foreffective operation. During operation of the EGB Filter, particulatepollutants are deposited onto the surface of the granules and theelectrical insulating properties of the granule/dust mixture can bedifferent from that of the clean granules. In fact, sodium sulfateexhibits electrical resistivity substantially lower than that of thegranules. The electrical resistivity of most solid materials is a strongfunction of temperature, resistivity decreasing with increasingtemperature.

It would be expected that as clean granules accumulate sodium sulfatedust, the bulk electrical resistivity of the filtration bed woulddecrease. FIG. 2 is a plot of electrical resistivity as a function oftemperature. Line (a) is representative of a typical granule used in EGBfilter system. As can be seen, resistivity strongly decreases withincreasing temperature, but is above the critical point of 1×10⁶ ohm-mfor temperatures of 450° C. or less. Line (b) is representative of thissame granule, but coated with sodium sulfate dust. As expected, theresistivity is reduced and now is above the critical resistivity fortemperatures below about 400° C. These data are based on laboratoryconditions, with samples heated in an oven.

In actual operation, the inventors have found that a quite differentsituation is encountered. Upon initial operation of the EGB filtersystem on glass furnace exhaust, for about the first 10-20 hours, dataof line (c) are obtained. At higher temperatures, greater than about450° C., the results are as expected. However, as temperature isdecreased, the resistivity falls well below the expected values, and infact decreases with decreasing temperature. This effect is typical offossil fuel combustion exhaust gas streams which contain acidcomponents. In this case, SO₃ is present and has the effect ofexhibiting an acid dew point. That is, upon cooling the flue gas, atsome temperature acid begins to condense onto solid surfaces. This isfurther aggravated by the fact that the sodium sulfate is veryhygroscopic, even deliquescent, which serves to elevate the acid dewpoint temperature above that which would be exhibited for acidcondensation onto clean inert surfaces. When the acid dew point isreached, acid condensation occurs which provides a new electricalconduction mechanism. This conduction has a different dependence ontemperature, thus the shape of line (c). Even with this acid dew pointeffect, the EGB filter technology can function, so long as temperaturesare maintained in a proper range where resistivity is above 10⁶ ohm-m.

However, upon extended operation, for example after several days ofoperation, some quite unexpected results are found. The resistivitycurve of the bed has now shifted from curve (c) to curve (d). The mostimportant difference is that at temperatures above the region of aciddew point, the resistivity is some 20 times less than that exhibited bythe bed during its initial operation. Analysis reveals that the cause isa change in the physical structure of the sodium sulfate dust on thegranule surface. Initially, the granule 40 is coated with sodium sulfatedust in the form of a particulate layer 41 as depicted in FIG. 3. Herethe main path of flow of electric current is through the layer of duston the granule. After the same granule is removed from the filter,cleaned of free dust, and returned to the filter several times, a solidlayer of sodium sulfate 42 forms on the granule surface, as depicted inFIG. 4. It can be theoretically demonstrated that such a solid layerexhibits electrical resistivity some 10-100 times lower than for aparticulate layer of the same material. Simply stated, in theparticulate layer, the electric current is forced to flow throughnumerous constrictions at the particle contact points, which increasesthe electrical resistivity compared to a solid layer. The actual size ofthe contact points between particles determines the actual value of theresistivity compared to that for the solid layer.

Conventional EGB filter systems installed on glass melting furnaces havefailed due to the above described effect. The initial electricalresistivity of the bed is in the acceptable range greater than 1×10⁶ohm-m, but after extended operation, the resistivity falls well belowthe acceptable value. Granules of such prior art processes removed fromthe operating filter and measured in the laboratory exhibit aresistivity curve as shown by (e). In the laboratory, tests areperformed under essential dry-acid free ambient air conditions, so theacid dew point effect is not present. These tests reveal that simplycooling the sample to temperatures below about 250° C. brings theresistivity into the acceptable range. Attempts to achieve acceptablefilter operation by gas cooling, as would be suggested by these tests,would fail, due to the effect acid dew point has on resistivity at theselower temperatures.

The inclusion of the prereactor bed 12 in the system, utilizing forexample calcium-based sorbent powder, results in successful operation ofthe electrostatic bed filter 18. By co-mingling the granules between twobeds, the solid coating on the granules is a mixture of CaSO₄ and Na₂SO₄. The electrical resistivity of CaSO₃ and CaSO₄ is several orders ofmagnitude greater than Na₂ SO₄. The resistivity of the granules coatedwith a solid layer of the mixture is shown as curve (f) in FIG. 2. Itcan be seen that a workable region of temperatures exists whereresistivity is greater than 1×10⁸ ohm-m.

The invention has the added benefit that SO₂ reduction can be quiteeffectively achieved in the prereactor bed. Although conventionaltechnology exists for reduction of SO₂ by dry injection of lime intoflue gases, the use of the moving bed prereactor substantially improvesthe effectiveness of the process, i.e., both improved SO₂ reduction andreduced lime injections requirements. The effect of the prereactor bedis to provide excellent gas/solid contact and extended residence time oflime in contact with the flue gas. HCl and HF gases are also effectivelyreduced with this technology.

In a second preferred embodiment, the EGB filter granules, rather thanbeing the above-described durable granules well known in EGB filtertechnology comprise a suitable alkaline earth metal sorbent. In thissecond embodiment, the need for a prereactor is eliminated since thegranules themselves supply the alkaline earth metal materials necessaryfor reaction with SO₃ and SO₂ in the flue gas.

Examples of alkaline earth metal materials suitable for forming the bedgranules are the same as those mentioned above in connection with theair-injected dust or powder for the prereactor. These include CaO,Ca(OH)₂, CaCO₃, MgO, Mg(OH)₂, MgCO₃, etc. The only limitations on thetypes of alkaline earth metal materials are that they be capable offorming granules and that they are sufficiently reactive to remove atleast a large portion of the sulfur containing gases from the exhaustgases.

The granules are obtained either in a mined natural form, such aslimestone (CaCO₃) or a manufactured agglomerate. The granules can beproduced by well known processes, such as pelletizing powdered alkalineearth metal materials, together with binders and fillers such as silicasand (for strength). Another useful process is briquetting, whichinvolves molding and compressing powdered alkaline earth metal materialswith binders and fillers.

As with the first embodiment, the appropriate size of the granules forthe second embodiment can be readily ascertained by one skilled in theart. For the second embodiment, the preferred size for the granules isfrom about 2.0 mm to about 20 mm and preferably from about 4.0 to about10.0 mm.

An apparatus suitable for use in this second embodiment is shown in FIG.5. The apparatus, indicated generally by the number 50, includes anelectrostatic preionizer 52 of conventional design where the entraineddust particles are given an electrostatic charge. Gases are thendirected to the electrostatic granular bed filter 54, again ofconventional design as exemplified by U.S. Pat. No. 4,338,113 which isherein incorporated by reference in its entirety. Dust particles areelectrostatically deposited onto the granules which fill the filter.Simultaneously, SO₂ and SO₃ gases are chemically reacted with thealkaline earth metal material to form solid sulfate and sulfite salt,and to release gaseous CO₂. Cleaned gases leave the filter at exit 56and a fan 58 is used to provide the underpressure to draw the gasesthrough the previously mentioned equipment. Gases then exit to theatmosphere through the stack 60.

Granules with attached dust particles are removed, either periodicallyor continuously, from the EGB filter 54 at its bottom outlet 62.Granules are directed to a means of mechanical agitation 64 sufficientto dislodge the alkali metal salts (e.g., Na₂ SO₄ dust) and reactionproduct alkaline earth metal sulfate (e.g., CaSO₄). Exemplary of wellknown mechanical agitation types useful in the invention include:

1. Tumbling mills where granular material is tumbled inside a rotatingdrum and impurities are dislodged by mechanical impact onto the drumwalls and other granules.

2. Fluidized beds where granular material is put into a fluidized stateby passing air vertically upward through a bed, and impurities aredislodged by mechanical impact between the granules.

3. Rotary abraders where granules are dropped into rotating wheel bladesand are thrown at high velocity against the casing walls, whereimpurities are dislodged.

4. High velocity air jets, where granules are entrained into a highvelocity air stream and are then impacted onto solid surfaces whereimpurities are dislodged.

After the impurities are mechanically dislodged from the granules, themixture is directed to a suitable classifier 46 where the fine material,typically less than 1 mm diameter, is separated from the granules, via avibrating screen or equivalent. The granules, which now exhibit freshlyexposed surfaces of alkaline earth metal material, are conveyed viatransport means, such as a bucket elevator 68, to the top of the filter54 where they are reintroduced through inlet 70. Fresh alkaline earthmetal material is added to the filter as required to make up formaterial lost in the agitation and screening steps. Fine sulfate salts(e.g. NaSO₄ and CaSO₄), and alkaline earth metal material from theclassifier 66 are directed to the glass melting furnace where it isincluded in the batch materials. This is only possible if the granularmaterial chosen for the filter is compatible with the glass batchingredients. Limestone, for example, is suitable for use with mostsoda-lime glass types.

The apparatus of FIG. 5 is effective at preventing the development of acontinuous solid layer of alkali metal salts (e.g., NaSO₄) on thegranules. This phenomenon can be seen most clearly with reference toFIGS. 6-8. FIG. 6 shows the surface of a fresh granule of an alkalineearth metal material such as CaCO₃. As shown in FIG. 7, after exposureto exhaust gases containing SO₂ and SO₃, a reaction product layer ofCaSO₄ 72 is formed. This layer 72 has a higher molar volume than CaCO₃(typically 1.5 times higher). As a result, the reaction product layer 72is loosely adhered to the granule surface. Subsequently, Na₂ SO₄ and/orother alkali metal salts 74 in dust form are deposited on top of thisloosely adhered reaction product, as shown in FIG. 8. The combined layeris then easily removed by the mechanical agitation and a fresh substrateof CaCO₃ is exposed.

The mechanical cleaning also exposes fresh sorbent surface on thealkaline earth metal material, which is then once again reactive withthe SO₂ and SO₃ in the exhaust gases. Thus the process has the addedbenefit of being effective at reduction of these gaseous pollutants inthe exhaust gases. The alkaline earth metal material is also reactivewith HF and HCl gases and their reduction can also be achieved.

Although the present invention has been described in connection withpreferred embodiments of the invention, it will be appreciated by thoseskilled in the art that additions, substitutions, modifications anddeletions not specifically described, may be made without departing fromthe spirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method for reducing pollution emission from aglass melting furnace exhaust stream which includes a gaseous sulfuroxide constituent, dust, and alkali metal salt particulates, comprisingthe steps of:(a) electrostatically ionizing dust and alkali metal saltparticulates in the exhaust gas stream; (b) providing an electrostaticgranular bed filter comprising groups, wherein the granules are solidalkaline earth metal material; (c) electrically polarizing the granularsolid alkaline earth metal material with a high voltage electrode; (d)passing the exhaust stream over the granular bed, thereby reacting thegranules with the sulfur-containing gases and forming alkaline earthmetal sulfates and sulfites on the surfaces of the granules; (e)simultaneously depositing dust and alkali metal salt particulates fromthe exhaust stream onto the granules; (f) removing the granular alkalineearth metal material from the bed; (g) mechanically agitating thegranules removed from the bed in step (f), thereby dislodging collecteddust, alkali metal salt particulates and alkaline earth metal sulfatesand sulfites from the granules and exposing a fresh surface of alkalineearth metal on the granules; (h) separating the granules from thecollected dust, alkali metal salt particulates, and alkaline earth metalsulfates and sulfites; and (I) recycling the separated granules to thebed.
 2. A method according to claim 1, wherein the granules are analkaline earth metal material selected from the group consisting ofCaCO₃, CaO, Ca(OH)₂, MgCO₃, MgO, Mg(OH)₂, and mixtures thereof.
 3. Amethod according to claim 1, wherein the alkaline earth metal granulescomprise CaCO₃.
 4. A method according to claim 1 wherein the collecteddust includes NaSO₄ and the alkaline earth metal surfaces and sulfitesinclude calcium sulfate and calcium sulfite.
 5. A method according toclaim 1, wherein the collected dust includes Na₂ SO₄ and the alkalineearth metal sulfate and sulfites are calcium sulfate and sulfite.
 6. Amethod according to claim 1, wherein the dislodged dust and alkalineearth metal sulfates and sulfites from the granules are recycled to theglass melting furnace as raw material feed.
 7. A method according toclaim 1, wherein the granules in step (b) range in size from about 2.0mm to about 20 mm.
 8. A method according to claim 1, wherein thegranules in step (b) range in size from about 4.0 mm about 10 mm.